The invention relates to a semiconductor diode, an electronic component and a voltage source converter. The invention furthermore relates to a control method for a voltage source converter.
With the aid of converters, an alternating-current system having a specific voltage, frequency and number of phases is converted into an alternating-current system having a different voltage, frequency and, if appropriate, number of phases. Voltage source converters use a double conversion method for the conversion. The input alternating current is firstly rectified. The DC voltage is smoothed in the intermediate circuit and converted into an alternating current having a different voltage and frequency in the inverter. Converters can furthermore be used for the conversion of systems in which a voltage regularly varies with respect to time without a voltage zero crossing occurring.
In voltage source converters appertaining to power electronics, components used include active semiconductor switches (turn-off power semiconductors, e.g. MOSFET (Metal Oxide Semiconductor Field Effect Transistor), IGBT (Isolated Gate Bipolar Transistor), bipolar transistors, GTO (Gate Turn-Off Thyristor), IGCT (Integrated Gate Commutated Thyristor)) and freewheeling diodes. Freewheeling diodes are diodes which can in electronic circuits appertaining to power electronics, during operations for turning off or switching over the electrical energy or charge stored in inductances, offer a current path for outputting energy or charge.
The switch-on speed of the turn-off power semiconductors is limited in voltage source converters by the time required by the freewheeling diode for it to be able to take up voltage. This in turn is limited by state delays on account of finite charge carrier speeds. This occurs particularly when current and voltage change very rapidly. The state delay results in particular from the fact that the diode, in the current-carrying phase, is flooded with charge carriers which, in the event of a phase change, that is to say switchover to the reverse direction, and the associated commutation, that is to say change in the current direction, firstly have to be depleted from the diode before the diode can take up voltage. The charge carriers that are still to be depleted during the switchover of the diode are also referred to as the storage charge and the associated behavior of the diode is referred to as the reverse recovery behavior.
Consequently, in voltage source converters, the reverse recovery behavior of the freewheeling diodes, in particular the time required for depleting the storage charge, limits the permissible switch-on speed of the active semiconductor switch. Before the diode can take up voltage, the storage charge has to be depleted. This causes a power loss both in the diode and in the semiconductor switch.
Reliable operation of the freewheeling diode can be ensured by means of a sufficiently slow switch-on speed of the active semiconductor switch. The power loss that arises has to be taken into account in the converter dimensioning. This leads to an increased cooling outlay or to an enlarged chip area of the power semiconductors or limits the operating frequency of the converter.
Hitherto, PIN diodes and—at lower voltages—Schottky diodes based on silicon have been used in voltage source converters.
Independently of and without reference to voltage source converters, various types of MOS (Metal Oxide Semiconductor)-controlled diodes (MCDs) are known from Schröder (Schröder, Dierk: “Elektrische Antriebe3—Leistungselektronische Bauelemente” [“Electrical drives 3—Power electronic components”], Springer-Verlag, Berlin, 1996, pages 373 to 377). Various types of MCDs are disclosed. In all of the MCDs described, a switchover between two states of the component is effected by means of a MOS control head, that is to say a gate electrode fitted in an insulated manner above the semiconductor material. Said states can be characterized as follows:
State 1: low forward resistance, high storage charge, blocking ability
State 2: high forward resistance, low or no storage charge, no or only little blocking ability
In state 1, all of the MCDs described behave like a PIN diode with a highly doped p-type region, that is to say that the component is readily conductive in the case of forward-biasing. Furthermore, it is capable of blocking, but a high storage charge has to be depleted in the transition from the forward direction to the reverse direction.
In state 2, the MCDs described behave, depending on the embodiment, like a switched on MOSFET or like a Schottky diode, that is to say poorer conductivity in the case of forward-biasing than in state 1, no or—on account of the Schottky contact—only little blocking ability, although also no or only low storage charge which has to be depleted when there is a change in the current direction.
All of the MCDs described are constructed in such a way that, as a result of the application of a gate voltage, a p- or n-doped semiconductor region is bridged by an n- or p-conducting channel. The switchover of the MCDs thus causes the conductive channel to be established or removed. In state 2, the pn junction is thereby “bypassed” by an alternative current path. The pn junction is therefore incapable of blocking in state 2. State 2 of the MCD is thus characterized by no or—in the case of behavior like a Schottky diode—by only little blocking ability.
Since state 1 has the lower forward resistance, this state should be set in the case of forward-biasing. In the case of reverse-biasing, the MCD can only be in state 1, since state 2 has no or only little blocking ability and can therefore take up no or only a low voltage. However, the MCD should be in state 2 where there is a change in the current direction, that is to say when there is a transition from the case of forward-biasing to the case of reverse biasing, since this state, in contrast to state 1, has no or only a low storage charge. When there is a change in the current direction, that is to say when there is a transition from the case of forward-biasing to the case of reverse-biasing, therefore, the MCD should—in order to achieve an optimum behavior of the MCD—initially be in state 1, and then be switched over to state 2, the change in the current direction should then be effected, and the switchover to state 1 should subsequently be effected, in order to realize the blocking.
What is disadvantageous about these MCDs described in Schröder is that the above-described method for realizing the optimum transition of the MCDs from the case of forward-biasing to the case of reverse-biasing is very complicated and reacts critically to the temporal sequence of the control pulses.